This invention relates to coating ceramic materials and more particularly to coating a fiber reinforced ceramic composite in which the adhesion of the coating to the ceramic composite is greatly improved, and to the coated ceramic composite thus produced.
The use of ceramic materials of monolithic or composite construction is common where high temperatures (e.g. 1000.degree. C. or higher) are encountered. Such ceramic materials are used in many applications in the aerospace industry including gas turbine parts, air frame components, rocket engine components, etc.
Major difficulties have been encountered with coating of ceramic materials. Proper adhesion of the coating to the ceramic can be difficult because stresses which develop due to varying degrees of thermal expansion of the ceramic and the coating. As a result, especially in high temperature applications, cracking of the coating frequently occurs allowing oxidation and a corrosive attack on the ceramic substrate. In addition, mechanical vibrations and other forms of physical stress or even debris damage may cause cracking and spalling of the protective coating layer if proper adhesion of the coating to the ceramic is not achieved.
Various attempts have been made in the art to relieve the foregoing problems associated with thermal expansion mismatch. Among the solutions includes coating the ceramic with an oxygen scavenging sealant layer to provide protection or a gradient in the thermal coefficient of expansion from the ceramic substrate to the outer oxidation resistant coating. A carbon body with an oxidation resistant coating is disclosed in U.S. Pat. No. 4,515,860 in which a coating is formed of a silicon alloy having a non-columnar grain distribution.
It is also well known in the art to clean various superalloys with different types of fluorides as is disclosed in U.S. Pat. Nos. 4,188,237 and 4,098,450. The art also discloses the cleaning or etching of various nonmetallic materials such as U.S. Pat. No. 3398033 which discloses etching a silicon surface with chlorine and oxygen, U.S. Pat. No. 3,465,421 which discloses cleaning a germanium surface and U.S. Pat. Nos. 3,607,473, 3,879,238 and 4,415,406 which disclose etching of a polymer article followed by metallizing or coating with a paint or adhesive. U.S. Pat. No. 4,615,763 discloses etching a fiber reinforced epoxy resin, with the etching only to roughen the surface, followed by coating with an adhesive then plating with copper or nickel in a desired pattern for use as a circuit breaker.
It is an object of this invention to provide a coating for a fiber reinforced ceramic composite.
It is another object of this invention to provide a coating with improved adhesion to the substrate of a fiber reinforced ceramic composite.